using FTPboxLib.dll

Removed Classes folder, using FTPboxLib.dll as the core library
Added FTPboxLib files, fixes #18

added a re-planned and partially re-written core library for FTPbox,
made with multi-platform support in mind.
updated .gitignore

also small update to csproj file
Added LICENSE
Added certificate support ( fixes #25 )

view (accept/decline) certificates, save trusted ones locally
Added certificate support on Windows